1. Field of the Invention
The present invention relates to a color image processing apparatus.
2. Description of the Related Art
Many conventional image processing techniques pay attention to a portion including a high spatial frequency component (i.e., an edge portion). For example, a typical processing technique is an edge emphasis technique for increasing sharpness of an image to improve image quality. On the other hand, in the field of an image compression technique, the way of processing the edge portion has a large influence on compression efficiency. More specifically, an image including an edge portion (high frequency component) has a large data volume. For example, when a lossless data storage type compression technique, such as a predictive coding technique or the like, is used, coding efficiency is impaired, and the overall compression ratio is decreased. On the other hand, when a lossy data non-storage compression technique such as a vector quantization technique or orthogonal transformation technique is employed, degradation of edge portions is considerable. In proportion an image includes more edge portions, quality of the reproduced image is degraded.
Therefore, a large number of adaptive type compression techniques have been proposed wherein image nature (edge portion versus flat portion) is determined for every specific area such as a block constituted by a plurality of pixels or a pixel of interest and its surrounding pixel group in an image, and a technique suitable for the determined type of image is employed for that block or pixel.
Edge portions which seem to be the same microscopically may often have different image tones macroscopically. In this case, if identical processing is performed for these portions, it may often be unsuccessful. More specifically, when edge emphasis is adopted as the processing, processing is preferably different for a line image such as a character image and for a halftone image such as a photograph. For example, in the case of edge emphasis, processing is preferably selectively performed in such a manner that relatively strong emphasis is performed for a line image, and relatively weak emphasis is performed (or no processing at all is made) for a halftone image.
When color image compression is adopted as the processing, in an adaptive compression/expansion technique depending on the presence/absence of an edge, data can be efficiently compressed in a color character image portion without image degradation. However, if such compression is performed for an edge substantially the same as a color character portion in the remaining color image, considerable image degradation occurs.